1. Field of the Invention
The present invention relates generally to satellite broadcasting receiving tuners (broadcasting satellite tuners) and, more particularly, to a satellite broadcasting receiving tuner provided with a means for preventing a received UHF signal from leaking out of the satellite broadcasting receiving tuner simultaneously with the output of a down-converted received satellite broadcasting signal.
2. Description of the Related Art
Conventionally, a signal receiving system such as that shown in FIG. 4 is used for the purpose of receiving two broadcast waves of ground wave broadcasting in the VHF and UHF bands and satellite broadcasting by different antennas and inputting the received waves to a television set.
Referring to FIG. 4, the signal receiving system has an antenna 31 for receiving ground wave broadcasts in the VHF and UHF bands (hereinafter referred to as "UV antenna") and a satellite broadcasting receiving antenna 32 (hereinafter referred to as "BS antenna"), which are installed outside a house, a booster 33 also installed outside the house (hereinafter referred to as "UV/BS booster"), a distributor 35 installed inside the house, a first coaxial cable 34 which connects an output terminal of the UV/BS booster 33 and an input terminal of the distributor 35, a second coaxial cable 36 which connects one of two output terminals of the distributor 35 and an antenna outlet 37, a third coaxial cable 38 which connects the other output terminal of the distributor 35 and another antenna outlet 39, and a UV/BS separator 40.
A ground wave broadcasting signal received by the UV antenna 31 (hereinafter referred to as "UV signal") and a satellite broadcasting signal received by the BS antenna 32 and frequency-converted by the down converter (LNB) 32a (hereinafter referred to as "BS signal") are respectively input to the UV/BS booster 33. The UV/BS booster 33 amplifies and mixes the input UV and BS signals to obtain a UV/BS signal, and outputs the UV/BS signal to the coaxial cable 34. The coaxial cable 34 leads the UV/BS signal from the exterior to the interior of the house to supply the signal to the distributor 35. The distributor 35 distributes the input UV/BS signal to the two output terminals. The UV/BS signal obtained at one of the two output terminals is supplied to the antenna outlet 37 through the coaxial cable 6 and is supplied from the antenna outlet 37 to the UV/BS separator 40. The UV/BS separator 40 separates the supplied UV/BS signal into UV and BS signals and separately outputs these signals. The UV signal is supplied to a UV receiving terminal of a television set 41 while the BS signal is supplied to a BS receiving terminal of the television set 41. The television set 41 has a UV tuner connected to the UV receiving terminal and a BS tuner connected to the BS receiving terminal. The UV tuner converts the input UV signal into an intermediate frequency signal (IF signal) and outputs the converted signal. The BS tuner also converts the input BS signal into an intermediate frequency signal (IF signal) and outputs the converted signal.
FIG. 5 is a block diagram showing the configuration of an example of the known BS tuner in the above-described system.
Referring to FIG. 5, a BS tuner 42 has an input terminal 43, a high-frequency amplifier (RF amplifier) section 44, a variable band-pass filter (BPF) 45, a mixer section 46, a local oscillator section 47, a first intermediate frequency amplifier (IF amplifier) section 48, a surface acoustic wave (SAW) filter 49, a second intermediate frequency amplifier (IF amplifier) section 50, and an output terminal 51. The input terminal 43 is connected to the BS receiving terminal of the television set 41.
The BS signal supplied to the input terminal 43 is amplified in the high-frequency amplifier section 44, and unnecessary frequency components in the BS signal are removed by the variable band-pass filter 45. Next, in the mixer section 46, the BS signal is frequency-mixed with a local oscillation signal supplied from the local oscillation section 46. An intermediate frequency signal in the frequency-mixed signal is amplified in the first intermediate frequency amplifier section 48. Subsequently, unnecessary frequency components of the intermediate frequency signal are removed by the surface acoustic wave filter 49, and the intermediate frequency signal is again amplified in the second intermediate frequency amplifier section 50 before being supplied to a subsequent circuit (demodulation circuit) through the output terminal 51.
FIG. 6A is a circuit diagram showing the configuration of an example of the variable band-pass filter 45 used in the BS tuner 42, and FIG. 6B is a diagram showing an equivalent circuit in a frequency range lower than the band-pass frequency of the variable band-pass filter 45. FIG. 7 is a diagram showing an example of frequency transmission characteristics of the variable band-pass filter 45.
As shown in FIG. 6A, the variable band-pass filter 45 has four strip lines 52 to 55 connected in series between an input terminal 45(I) and an output terminal 45(O), a strip line 56 connected as a branch line between a point of connection of the strip lines 53 and 54 and a ground point, a capacitor 57 and a variable capacitance diode 58 connected in series between a point of connection of the strip lines 52 and 53 and a ground point, a capacitor 59 and a variable capacitance diode 60 connected in series between a point of connection of the strip lines 54 and 55 and a buffer resistor 62 connected between a point of connection of the capacitor 57 and the variable capacitance diode 58 and a tuning control voltage supply terminal 61, a buffer resistor 63 connected between a point of connection of the capacitor 59 and the variable capacitance diode 60 and the tuning control voltage supply terminal 61.
A predetermined tuning control voltage is supplied to the tuning control voltage supply terminal 61 to apply bias voltages to the variable capacitance diodes 58 and 60. The capacitances of the variable capacitance diodes 58 and 60 are changed by the bias voltages to tune the variable band-pass filter 45 so that the band-pass frequency of the filter coincides with a predetermined frequency in the range from 1 to 2 GHz (1000 to 2000 MHz), i.e., the frequency of the input BS signal. At this time, the BS signal supplied to the input terminal 45(I) passes the variable band-pass filter 45 while unnecessary components thereof are removed. The BS signal is thereafter supplied from the output terminal 45(O) to the subsequent circuit.
For example, in the case of a European broadcasting system, a BS wave in the 12 GHz band may be received by the BS antenna 32 and is converted into a BS signal in the band from 1 to 2 GHz by the down converter 32a provided in the vicinity of the BS antenna 32. The BS signal is supplied to the BS receiving terminal of the television set 41 via the UV/BS booster 33, the distributor 35 and the UV/BS separator 40. Next, in the television set 41, the BS signal applied to the BS receiving terminal is supplied to the BS tuner 42 to undergo high-frequency amplification, frequency conversion and intermediate frequency amplification. Thereafter, the BS signal is supplied as an intermediate frequency signal from the BS tuner 42 to the subsequent circuit.
The UV/BS signal supplied to the UV/BS separator 40 in this receiving is separated into UV and BS signals by the UV/BS separator 40. Ordinarily, the BS signal output from the UV/BS separator 40 to the BS receiving terminal of the television set 41 contains a UV signal component at a comparatively high level because the signal separating ability of the UV/BS separator 40 is not sufficiently high.
Assuming that the frequency band of the BS signal input to the BS tuner 42 is from 1 to 2 GHz (1000 to 2000 MHz) as mentioned above and that the frequency band of the intermediate frequency of the BS tuner 42 is 480 MHz, a television broadcasting wave having a frequency of 480 MHz in television broadcasting waves in the UHF band having frequencies of 470 to 480 MHz coincides with the 480 MHz intermediate frequency signal of the BS tuner 42.
It the BS signal output from the UV/BS separator 40 is applied to the BS receiving terminal of the television set 41 with the 480 MHz television broadcasting wave contained therein and is then supplied to the BS tuner 42, then the 480 MHz television broadcasting wave is first amplified in the high-frequency amplifier section 44 of the BS tuner 42 and is then supplied to the variable band-pass filter 45.
At this time, since in the known variable band-pass filter 45 the 480 MHz television broadcasting wave (disturbing wave) corresponds to the non-transmission range (band-stop range) of the variable band-pass filter 45, it is essentially necessary that the disturbing wave component having a frequency of 480 MHz be sufficiently attenuated and removed by the variable band-pass filter 45 so as not to be transmitted to the output terminal of the variable band-pass filter 45 and to the output terminal 51 of the BS tuner 42. However, residual disturbing wave components having a frequency of 480 MHz are output through the output terminal 51 of the BS tuner 42 for various reasons described below.
First, in the variable band-pass filter 45, the reactance of the variable capacitance diodes 58 and 60 becomes large in a frequency range lower than the band-pass frequency. With respect to frequencies in the vicinity of 500 MHz, each of the variable capacitance diodes 58 and 60 is regarded as being in a substantially open condition such that an equivalent circuit shown in FIG. 6B is formed. The characteristics of the variable band-pass filter 45 are such that, as shown in FIG. 7, an attenuation in the variable band-pass filter 45 at a frequency in the vicinity of 480 MHz is about 16 db when the band-pass filter 45 is tuned to 1 GHz (1000 MHz), and that, even when the variable band-pass filter 45 is tuned to 2 GHz (2000 MHz), an attenuation in the variable band-pass filter 45 at a frequency in the vicinity of 480 MHz is only slightly larger than 16 db. Therefore, the 480 MHz television broadcasting wave is not sufficiently attenuated by the variable band-pass filter 45, so that part of the 480 MHz television broadcasting wave is output from the variable band-pass filter 45. Second, when the 480 MHz television broadcasting wave is amplified by the high-frequency amplifier section 44, part of the 480 MHz television broadcasting wave is converted into a disturbing wave component having a frequency of about 1 GHz (1000 MHz) by secondary distortion in the high-frequency amplifier section 44. This disturbing wave component passes the variable band-pass filter 45 to be contained in the output from this filter. Third, a disturbing wave component having a frequency in the vicinity of 480 MHz is generated in the mixer section 46. This disturbing wave component is not completely removed by amplification in the first intermediate frequency amplifier section 48 and in the second intermediate frequency amplifier section 50. Also, this disturbing wave component cannot be completely removed by the surface acoustic wave filter 49. As a result, this component is supplied to the output terminal 51. In these disturbing wave components, the amplitude of the disturbing wave with the first reason is largest while the amplitude of the disturbing wave with the third reason is smallest.
The known variable band-pass filter 45 has a drawback described below. Because stray capacitances Cs (e.g., about 0.5 pF) of the buffer resistors 62 and 63 for supplying the tuning control voltage to the variable capacitance diodes 58 and 60 are connected in parallel with the capacitances of the variable capacitance diodes 58 and 60, it is difficult to tune the variable band-pass filter 45 to the high-end frequency of 2 GHz (2000 MHz) of the BS signal. As shown in FIG. 7, the minimum value of the frequency attenuation characteristic of the variable band-pass filter 45 is at a frequency slightly lower than 2 GHz when the variable band-pass filter 45 is controlled for tuning to the high-end frequency of 2 GHz.
As described above, disturbing wave components having frequencies in the vicinity of 480 MHz cannot be completely removed from the output from the band-pass filter 45 used in the known BS tuner 42 and it is difficult to tune the variable band-pass filter 45 to the high-end frequency.